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Influence of Cation Structure on Binary Liquid-Liquid Equilibria for Systems Containing Ionic Liquids Based on Trifluoromethanesulfonate Anion with Hydrocarbons Andrzej Marciniak* and Ewa Karczemna Department of Physical Chemistry, Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ReceiVed: February 2, 2010
Binary liquid-liquid equilibria for 15 systems containing an ionic liquid (1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylpyridinium trifluoromethanesulfonate, 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by the dynamic method. The influence of cation structure of trifluoromethanesulfonate anion based ionic liquids on solubility of aliphatic and aromatic hydrocarbons is discussed. Introduction Ionic liquids (ILs) are a relatively new class of salts with a melting temperature below 100 °C.1 In general ILs are composed of organic cations with either inorganic or organic anions. The vapor pressure of ionic liquids is negligible, which makes them good replacements for conventional volatile often flammable, and toxic organic solvents in the chemical industry, especially as entrainers in separation processes. Additionally, ionic liquids have other important properties required for entrainers, namely a wide liquid range and stability at high temperatures. From activity coefficients at infinite dilution measurements it was shown that a large number of ionic liquids have better selectivity and capacity in extraction of aromatics from aromatic/aliphatic mixtures than typical solvents such as sulfolane and N-methylpyrrolidone (NMP).2 The investigated ionic liquids, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][CF3SO3]), 1-butyl-3-methylpyridinium trifluoromethanesulfonate ([1,3bmPY][CF3SO3]), and 1-butyl1-methylpyrrolidinium trifluoromethanesulfonate ([bmPYR][CF3SO3]), show higher values of selectivity and capacity than sulfolane in the n-hexane/benzene separation problem.3-5 To design an ionic liquid for a specific separation problem the knowledge of the influence of cation and anion structure on solubility is required. Ionic liquids are considered as extractants in the separation of sulfur compounds from fuels,6-11 therefore the solubility of aromatic and aliphatic hydrocarbons in ionic liquids for this problem is very helpful and important. In this paper the influence of the cation structure of trifluoromethanesulfonate anion based ionic liquids on the solubility of aliphatic and aromatic hydrocarbons is presented. Binary liquid-liquid equilibria for 15 systems containing an ionic liquid (1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylpyridinium trifluoromethanesulfonate, 1-butyl-1methylpyrrolidinium trifluoromethanesulfonate) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by the dynamic method. Experimental Methods Materials. The ionic liquid [bmim][CF3SO3] had a purity of >0.999 mass fraction and was supplied by Merck. The ionic * To whom correspondence should be addressed. E-mail: a.marciniak@ ch.pw.edu.pl. Fax: +48-22-628 2741. Phone: +48-22-234 5816.
liquid [bmPYR][CF3SO3] had a purity of >0.99 mass fraction and was supplied by Merck. The ionic liquid [1,3bmPY][CF3SO3] had a purity of >0.99 mass fraction and was supplied by IoLiTec. The ionic liquids were further purified by subjecting the liquid to a very low pressure of about 5 × 10-3 Pa at a temperature of about 95 °C for ca. 5 h. This procedure removed any volatile chemicals and water from the ionic liquid. The list of hydrocarbons used in this study including source and grade is as follows: n-hexane, Fluka, grade >0.997 mass fraction; n-heptane, Sigma-Aldrich, grade >0.995 mass fraction; cyclohexane, Sigma-Aldrich, grade >0.997 mass fraction; benzene, Sigma-Aldrich, grade g0.999 mass fraction; toluene, SigmaAldrich, grade g0.999 mass fraction. While the purity of hydrocarbons was high all hydrocarbons were used without further purification. Water Content. The water content was analyzed by Karl Fischer titration technique (method TitroLine KF). A sample of IL was dissolved in methanol and titrated with steps of 2.5 µL. The results obtained have shown the water content to be less than 100 ppm. Liquid-Liquid Phase Equilibria Apparatus and Measurements. Two phases of disappearance observed with an increasing temperature have been determined by using a dynamic (synthetic) method described previously.12-14 The compound was kept under nitrogen in a drybox. Mixtures of solute and solvent were prepared by weighing the pure components to within 10-4 g. The sample of solute and solvent was heated very slowly (at less than 2 deg · h-1 near the equilibrium temperature) with continuous stirring inside a Pyrex glass cell, placed in a thermostat. The foggy solution disappearance temperature detected visually was measured with a calibrated electronic thermometer P 550 (DOSTMANN electronic GmbH). The measurements were carried out over a wide range of solute mole fraction ranging from 10-4 to 1. The uncertainty of temperature measurements was (0.05 K, and that of the mole fraction did not exceed (0.0002. The reproducibility of the LLE experimental points was (0.1 K. The experimental results are listed in Table 1. Results For this study, the liquid-phase behavior for 15 binary ionic liquid-hydrocarbon systems was determined. Results are presented in the Table 1 and in Figures 1-3. The structures of
10.1021/jp100994d 2010 American Chemical Society Published on Web 04/01/2010
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TABLE 1: Experimental Binary Liquid-Liquid Equilibria for Systems {Ionic Liquid (1) + Hydrocarbon (2)} x1
T/K
x1
T/K
x1
T/K
0.9678 0.9643 0.9600
[bmim][CF3SO3] + n-hexane 291.5 0.9556 311.3 0.9448 298.1 0.9528 316.7 0.9418 304.8 0.9478 323.7 0.9382
329.6 333.9 339.4
0.9811 0.9776 0.9726
[bmim][CF3SO3] + n-heptane 299.5 0.9682 336.4 0.9611 310.9 0.9656 342.4 0.9569 324.6 0.9642 346.5 0.9528
353.8 361.9 370.6
0.9423 0.9339 0.9269
[bmim][CF3SO3] + cyclohexane 295.1 0.9205 321.6 0.9031 305.1 0.9153 329.3 0.8951 313.8 0.9065 337.8 0.8925
342.1 350.1 352.6
0.2745 0.2775
296.7 306.4
[bmim][CF3SO3] + benzene 0.2838 325.0 0.2900 0.2881 339.2 0.2921
345.3 352.2
0.4403 0.4419 0.4429
301.4 302.3 305.9
[bmim][CF3SO3] + toluene 0.4437 307.7 0.4591 0.4473 315.2 0.4630 0.4561 341.2
0.9549 0.9522
[1,3bmPY][CF3SO3] + n-hexane 303.0 0.9480 314.4 0.9387 307.6 0.9433 322.3 0.9327
0.9660 0.9620 0.9564
[1,3bmPY][CF3SO3] + n-heptane 301.8 0.9527 327.4 0.9440 308.6 0.9499 336.0 0.9404 319.6 0.9466 343.5
349.6 359.2
0.9044 0.9006 0.8962 0.8928
[1,3bmPY][CF3SO3] + cyclohexane 304.4 0.8899 324.4 0.8749 309.8 0.8862 329.1 0.8727 316.3 0.8818 333.7 0.8701 320.4 0.8785 338.4
343.3 347.3 351.0
0.2276 0.2287 0.2298 0.2308 0.2315 0.2324 0.2334
[1,3bmPY][CF3SO3] + benzene 282.9 0.2351 307.3 0.2442 285.9 0.2363 310.6 0.2467 289.6 0.2372 313.0 0.2453 293.1 0.2386 318.1 0.2456 295.7 0.2413 323.3 0.2476 299.0 0.2427 331.0 0.2467 302.5 0.2431 331.4
337.8 342.0 345.1 346.0 348.0 348.4
0.3601 0.3614 0.3633 0.3653 0.3668
[1,3bmPY][CF3SO3] + toluene 285.9 0.3679 303.0 0.3752 288.2 0.3696 307.0 0.3763 291.2 0.3714 310.9 0.3770 295.0 0.3728 315.7 0.3781 298.6 0.3744 320.5
325.9 329.7 332.6 360.2
0.9641 0.9619 0.9562
[bmPYR][CF3SO3] + n-hexane 292.7 0.9533 311.0 0.9416 296.8 0.9483 319.3 0.9375 306.2 0.9453 324.1 0.9357
0.9791 0.9744 0.9683
[bmPYR][CF3SO3] + n-heptane 294.7 0.9633 335.8 0.9531 305.2 0.9614 342.0 0.9495 324.7 0.9566 352.3
358.7 366.2
0.9307 0.9207 0.9137
[bmPYR][CF3SO3] 302.0 0.9061 311.9 0.8996 323.1 0.8927
+ cyclohexane 331.4 0.8841 340.3 346.2
353.7
0.2703 0.2719 0.2730 0.2744 0.2759
[bmPYR][CF3SO3] + benzene 289.4 0.2777 312.5 0.2847 293.8 0.2798 317.9 0.2860 297.7 0.2814 322.5 0.2867 302.6 0.2830 326.3 0.2876 308.0 0.2838 329.5
333.9 339.1 344.0 353.0
0.4179 0.4197 0.4212 0.4231 0.4244
[bmPYR][CF3SO3] + toluene 286.0 0.4265 308.1 0.4357 289.5 0.4286 313.8 0.4375 293.9 0.4306 318.5 0.4394 298.2 0.4326 323.4 0.4421 302.9 0.4340 328.6
333.7 340.3 347.5 365.3
350.6 370.9
Figure 1. Liquid-liquid phase equilibria of {[bmim][CF3SO3] (1) + hydrocarbon (2)} binary systems: (b) n-hexane; (9) n-heptane; (2) cyclohexane; (O) benzene; (0) toluene. Solid lines are calculated by means of the NRTL equation.
330.4 339.9
330.4 337.1 340.2
Figure 2. Liquid-liquid phase equilibria of {[1,3bmPY][CF3SO3] (1) + hydrocarbon (2)} binary systems: (b) n-hexane; (9) n-heptane; (2) cyclohexane; (O) benzene; (0) toluene. Solid lines are calculated by means of the NRTL equation.
cations and anion are presented below to visualize difference in cation structure of the investigated ionic liquids:
The liquid-liquid equilibria diagrams have shapes typical for diagrams with upper critical solution temperature (UCST) for n-hexane, n-heptane, and cyclohexane and with lower critical solution temperature (LCST) for benzene and toluene. Similar diagrams with UCST for aliphatic hydrocarbons were observed for other ionic liquids.12,15-22 The miscibility gap increases with an increase of the length of the alkane chain. Cyclohexane has better solubility in all investigated ionic liquids than n-hexane. This shows that aliphatic hydrocarbons with cyclic structure reveal better solubility than linear alkanes (similia similibus solVuntur). Aromatic hydrocarbons, benzene and toluene, have much better solubility in ionic liquids than aliphatic hydrocarbons. This is the result of
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Figure 3. Liquid-liquid phase equilibria of {[bmPYR][CF3SO3] (1) + hydrocarbon (2)} binary systems: (b) n-hexane; (9) n-heptane; (2) cyclohexane; (O) benzene; (0) toluene. Solid lines are calculated by means of the NRTL equation.
Figure 5. Liquid-liquid phase equilibria of {ionic liquid (1) + n-heptane (2)} binary systems: (O) [bmim][CF3SO3]; (∆) [1,3bmPY][CF3SO3]; (0) [bmPYR][CF3SO3]. Solid lines are calculated by means of the NRTL equation.
Figure 4. Liquid-liquid phase equilibria of {ionic liquid (1) + n-hexane (2)} binary systems: (O) [bmim][CF3SO3]; (∆) [1,3bmPY][CF3SO3]; (0) [bmPYR][CF3SO3]. Solid lines are calculated by means of the NRTL equation.
Figure 6. Liquid-liquid phase equilibria of {ionic liquid (1) + cyclohexane (2)} binary systems: (O) [bmim][CF3SO3]; (∆) [1,3bmPY][CF3SO3]; (0) [bmPYR][CF3SO3]. Solid lines are calculated by means of the NRTL equation.
the interaction between six π-delocalized electrons in the aromatic structure with the polar ionic liquids. Toluene with more aliphatic character caused by the presence of a methyl group in the structure shows reduction of solubility. These conclusions are coherent with results obtained previously. In general compounds with more aliphatic character are less soluble in ionic liquids. As was mentioned above, systems with aromatic hydrocarbons show lower critical solution temperature. This is not typical behavior, but was reported in the literature. Several LLE diagrams with LCST can be found in the literature, namely, [emim][SCN], [bmim][SCN], and [hmim][SCN] + thiophene;23 [bmim][SCN] + benzene, toluene, ethylbenzene, and tetrahydrofurane;17 [hmim][SCN] + benzene, toluene, and ethylbenzene;18 and [emim][NTf2] + benzene and R-methylstyrene.24 Figures 4-8 show the influence of an ionic liquid cation structure on the solubility of the hydrocarbons. For each hydrocarbon the solubility in the trifluoromethanesulfonate anion based ionic liquid takes the following order: [1,3bmPY]+ > [bmPYR]+ > [bmim]+. The differences in solubility for aliphatic hydrocarbons are not so high as those for aromatic ones. The same alkyl substituents (methyl and butyl) in each cation do not strongly affect the solubility of aliphatic hydrocarbons. The
more aliphatic character of the pyrrolidinium cation causes slightly better solubility of aliphatics than the imidazolium one. Surprisingly the pyridinium cation with the most aromatic character causes better solubility of aliphatic hydrocarbons. The packing effect and van der Waals interaction play the key role in this case. As was expected, the most aromatic pyridinium cation causes much better solubility of benzene and toluene than pyrrolidinium and imidazolium ones. The similar structure of this cation to the benzene structure additionally improves the solubility. The imidazolium cation with aromatic character does not improve the solubility of aromatic hydrocarbons in comparison to the more aliphatic pyrrolidinium cation. In the aromatics/aliphatics separation process a very low solubility of aliphatic hydrocarbons and high solubility of aromatic hydrocarbons in ionic liquid is required. At a given temperature of 323.15 K the solubility ratio of benzene and n-hexane is 0.25 for [1,3bmPY]+ and 0.30 for [bmPY]+ and [bmim]+ cations. For benzene and cyclohexane the solubility ratio at the same temperature is 0.27 for [1,3bmPY]+ and 0.31 for [bmPY]+ and [bmim]+ cations. The solubility ratio at a temperature of 323.15 K of toluene and n-heptane is as follows:
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J. Phys. Chem. B, Vol. 114, No. 16, 2010 5473 0.39 for [1,3bmPY]+, 0.45 for [bmPY]+, and 0.46 for [bmim]+ cations. It shows that ionic liquids [bmim][CF3SO3] and [bmPYR][CF3SO3] have higher selectivity in the aromatics/ aliphatics separation process than [1,3bmPY][CF3SO3]. Modeling. The LLE was correlated with the NRTL model describing the excess Gibbs energy.25 The equations were described by us earlier.26,27 The NRTL nonrandom parameter R was set to a value of R ) 0.1, which has given the best results of the correlations. For LLE, the temperature-dependent model adjustable parameters g12 - g22 ) a12 + b12T and g21 - g11 ) a21 + b21T were found by minimization of the objective function (OF): n
OF )
∑ [(∆x1)i2 + (∆x1*)i2]
(1)
i)1
Figure 7. Liquid-liquid phase equilibria of {ionic liquid (1) + benzene (2)} binary systems: (O) [bmim][CF3SO3]; (∆) [1,3bmPY][CF3SO3]; (0) [bmPYR][CF3SO3]. Solid lines are calculated by means of the NRTL equation.
where n is the number of experimental points and ∆x is defined as
∆x ) xcalcd - xexptl
(2)
The root-mean-square deviation of mole fraction for the LLE calculations was defined as follows:
σx )
(
n
∑ i)1
∑
)
1/2
n
(∆x1)i2 +
(∆x1*)i2
i)1
2n - 2
(3)
By analogy to the previous experiments (spectroscopic measurements),17 it was assumed in this work that the solubility at the solvent-rich phase was in the range of x1 ) 10-5 in n-hexane, n-heptane, and cyclohexane and x1 ) 5 × 10-5 in benzene and toluene. The results of the correlations, values of the model parameters, and the corresponding standard deviations are given in Table 2. For the systems presented in this work the average root-mean-square deviation σx is less than 0.0004. The results of the correlations are presented in Figures 1-3.
Figure 8. Liquid-liquid phase equilibria of {ionic liquid (1) + toluene (2)} binary systems: (O) [bmim][CF3SO3]; (∆) [1,3bmPY][CF3SO3]; (0) [bmPYR][CF3SO3]. Solid lines are calculated by means of the NRTL equation.
Conclusions The knowledge of the impact of cation structure on the liquid phase behavior of ionic liquids with hydrocarbons is useful for
TABLE 2: Correlation of the LLE Data by Means of the NRTL Equationa and the Mole Fraction Deviations σxb g12-g22/J · mol-1
a
g21-g11/J · mol-1
system
a12
b12
a21
b21
σx
[bmim][CF3SO3] + n-hexane [bmim][CF3SO3] + n-heptane [bmim][CF3SO3] + cyclohexane [bmim][CF3SO3] + benzene [bmim][CF3SO3] + toluene [1,3bmPY][CF3SO3] + n-hexane [1,3bmPY][CF3SO3] + n-heptane [1,3bmPY][CF3SO3] + cyclohexane [1,3bmPY][CF3SO3] + benzene [1,3bmPY][CF3SO3] + toluene [bmPYR][CF3SO3] + n-hexane [bmPYR][CF3SO3] + n-heptane [bmPYR][CF3SO3] + cyclohexane [bmPYR][CF3SO3] + benzene [bmPYR][CF3SO3] + toluene
-38.05 -34.75 -37.89 -41.64 -30.81 -32.85 -29.47 -31.50 -45.92 -37.49 -35.62 -32.80 -36.58 -42.86 -32.41
10752 11349 9473 -3478 -2496 8795 8394 6529 -3734 -2174 9838 10308 8836 -3208 -2379
128.3 126.4 135.3 134.2 121.3 132.2 123.4 128.6 141.6 130.6 132.0 125.4 136.4 137.8 123.0
-8902 -9982 -9845 4651 3143 -9747 -6826 -6685 5276 2714 -9860 -9222 -9749 4402 3182
0.0003 0.0001 0.0005 0.0002 0.0004 0.0001 0.0005 0.0003 0.0004 0.0010 0.0001 0.0002 0.0006 0.0004 0.0004
Parameters: g12 - g22 ) a12 + b12T and g21 - g11 ) a21 + b21T. b Parameter: R ) 0.1.
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developing ionic liquids as “designer solvents” for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures. From this work, it was shown how the liquid-liquid phase behavior of trifluoromethanesulfonate anion based ionic liquids with hydrocarbons is influenced by the cation structure. Pyridinium cation causes better solubility of both aliphatic and aromatic hydrocarbons than pyrrolidinium and imidazolium ones. The similar structure of this cation to the benzene causes better solubility. From the LLE experiments it is clear that ionic liquids [bmim][CF3SO3] and [bmPYR][CF3SO3] have higher selectivity in the aromatics/aliphatics separation process than [1,3bmPY][CF3SO3]. All systems were modeled by using the NRTL equation, with linear temperature-dependent adjustable parameters providing an excellent fit of the experimental data. Acknowledgment. Funding for this research was provided by the Warsaw University of Technology. The authors would like to thank Prof. Urszula Doman´ska for very helpful discussion and guidance. References and Notes (1) Crosthwaite, J. M.; Muldoon, M. J.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2006, 110, 9354–9361. (2) Marciniak, A. Fluid Phase Equilib. DOI:10.1016/j.fluid.2009.12.025. Published Online: Dec 29, 2009. (3) Doman´ska, U.; Marciniak, A. J. Phys. Chem. B 2008, 112, 11100– 11105. (4) Doman´ska, U.; Redhi, G. G.; Marciniak, A. Fluid Phase Equilib. 2009, 278, 97–102. (5) Marciniak, A.; Wlazło, M. J. Chem. Eng. Data, published online March 22, http://dx.doi.org/10.1021/je1000582. (6) Holbrey, J. D.; Lo´pez-Martin, I.; Rothenberg, G.; Seddon, K. R.; Silvero, G.; Zheng, X. Green Chem. 2008, 10, 87–92.
Marciniak and Karczemna (7) Alonso, L.; Arce, A.; Francisco, M.; Rodrı´guez, O.; Soto, A. AIChE J. 2007, 53, 3108–3115. (8) Alonso, L.; Arce, A.; Francisco, M.; Soto, A. J. Chem. Thermodyn. 2008, 40, 966–972. (9) Alonso, L.; Arce, A.; Francisco, M.; Soto, A. J. Chem. Thermodyn. 2008, 40, 265–270. (10) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494–2495. (11) Esser, J.; Wasserscheid, P.; Jess, A. Green Chem. 2004, 6, 316– 322. (12) Doman´ska, U.; Marciniak, A. J. Chem. Eng. Data 2003, 48, 451– 456. (13) Doman´ska, U.; Marciniak, A. J. Phys. Chem. B 2004, 108, 2376– 2382. (14) Doman´ska, U.; Marciniak, A.; Królikowski, M. J. Phys. Chem. B 2008, 112, 1218–1225. (15) Doman´ska, U.; Kro´likowski, M. J. Chem. Thermodyn. 2010, 42, 355–362. (16) Doman´ska, U.; Kro´likowski, M.; Pobudkowska, A.; Letcher, T. M. J. Chem. Eng. Data 2009, 54, 1435–1441. (17) Doman´ska, U.; Laskowska, M.; Pobudkowska, A. J. Phys. Chem. B 2009, 113, 6397–6404. (18) Doman´ska, U.; Kro´likowska, M.; Arasimowicz, M. J. Chem. Eng. Data 2010, 55, 773-777. (19) Doman´ska, U.; Marciniak, A. Green Chem. 2007, 9, 262–266. (20) Doman´ska, U. Pure Appl. Chem. 2005, 77, 543–557. (21) Doman´ska, U.; Marciniak, A. J. Chem. Thermodyn. 2005, 37, 577– 585. (22) Doman´ska, U.; Marciniak, A.; Bogel-Łukasik, R. ACS Symp. Ser. 2005, 901, 256–269. (23) Doman´ska, U.; Kro´likowski, M.; S´lesin´ska, K. J. Chem. Thermodyn. 2009, 41, 1303–1311. (24) Łachwa, J.; Szydłowski, J.; Makowska, A.; Seddon, K. R.; Esperanc¸a, J. M. S. S.; Guedes, H. J. R.; Rebelo, L. P. N. Green Chem. 2006, 8, 262–267. (25) Renon, H.; Prausnitz, J. M. AIChE J. 1986, 14, 135–144. (26) Doman´ska, U.; Marciniak, A. Green Chem. 2007, 9, 262–266. (27) Doman´ska, U.; Marciniak, A. Fluid Phase Equilib. 2005, 238, 137–141.
JP100994D
